Field of the Invention
The present invention relates to control of reactive power at a renewable energy site. More particularly, the present invention relates to improvements on traditional renewable energy site reactive power and voltage control systems subject to utility voltage and reactive power limits and significant control loop delay.
Description of Related Art
Renewable energy sites are typically composed of multiple power conversion devices connected in parallel generating fixed frequency AC power to a grid. The devices are typically AC-AC or DC-AC inverters. Inverters are designed to extract maximum power from the renewable power supply, subject to a real power limit reference and often, a reactive power or voltage command.
A typical reactive power control system measures site total power feedback, reactive power and site voltage to actively control them. The site control loop consists of commands from the controller to the inverters and feedbacks from the inverters or a utility meter to the site controller. Reactive power control runs concurrently and relatively independently of real power control. The site controller typically generates a site level reactive power command and divides this by the number of online inverters to obtain individual inverter commands. A reactive power controller regulates site voltage or power factor, but not both at once since voltage and reactive power are mutually dependent. Furthermore, reactive power and voltage commands are subject to site voltage and power factor operating limits. Local reactive power controllers residing in inverters are typically much faster than the remote site control loop. Therefore it is important to implement as much control functionality by the inverter itself, if possible.
The system overview for an example system with 4 inverters is shown in FIG. 1. The site control loop 10 consists of commands 23 from the controller 12 to the inverters 18 and feedbacks 21 from the inverters 18 or a utility meter 14 back to the site controller 12. The inverters 18 are connected to a power source 16 such as a photovoltaic (PV) module, and a step-up transformer 20 may intervene between the inverters 18 and the power meter 14. A point-of-control (POC) 15 is located next to the power meter 14 before the point of interconnection (POI) 17 with the utility.
The traditional controller is composed of an inner voltage control loop and an outer reactive power loop. FIG. 2 shows an example of a traditional controller 30. The inner voltage loop generates a reactive power command for the site (QCOMSITE) 59 that may be converted to a reactive power command for each inverter (QCOMINVERTER) 65 by division 64 by the number of inverters online 61. QCOMINVERTER 65 maintains voltage at either a fixed voltage reference (VREF) 41, or a dynamic voltage reference (VCOM) 39. The choice of voltage reference 45 is determined by the reactive power mode 42: voltage control (VREF 41) or power factor control (VCOM 39). Power factor control feeds the error (QERR) 37 resulting from subtraction 32 of the reactive power feedback (QFBK) 31 from the sum of the reactive power compensation (QCOMP) 35 and the reactive power reference (QREF) 33 (generated from a power factor reference) into a PI controller 38 to generate the dynamic voltage reference (VCOM) 39. Thus, this controller offers a way to control voltage or power factor, depending on the reactive power mode. The voltage reference 45 is subject to voltage limits 46, and the voltage feedback 49 is subtracted 48 from the limited voltage reference 47 to generate a voltage error (VERR) 51. The voltage error (VERR) 51 is fed into a PI controller 52 to generate a reactive power command 55 which is subject to limits 56 before the site reactive power command (QCOMSITE) 59 is generated.
However, traditional controllers can be improved significantly. The following shortcomings are present in a typical two-loop site controller.
1. Instability in reactive power control mode due to loop phase lag and delay. In reactive power control mode, the two series PI controllers can contribute excessive phase lag when the P gain is low, which decreases controller stability. (The classical two-loop technique is beneficial when the inner loop (voltage control, in this case) has a much faster response than the outer loop. However there is minimal benefit in this case, since voltage and reactive power are mutually dependent).
2. Large transients occurring when switching control modes or breaching voltage or reactive power thresholds, or when changing references, due to loop delay.
3. There is no means to apply reactive power threshold control during voltage control. Although the traditional two-loop structure conveniently implements voltage limits during power factor control, it does not impose reactive power limits during voltage control.
Various power controllers have been disclosed, such as those described in U.S. Pat. No. 7,923,862, U.S. Pat. No. 7,890,217, U.S. Pat. No. 6,512,966, and U.S. Published Patent Application No. 2010/0145532, have failed to overcome the limitations described herein. Thus, there is a need for an improved method of renewable power plant reactive power control with improved dynamic performance.